Solution for forming zinc phosphate coatings on metallic surfaces



United States Patent 3,161,549 SULUTKGN Fflli FflRMlNG ZiNC PHOSPHATE CUATHJGS 0N METALLRI SURFACES Rudolf Kallenhach, Windsor, @ntas'io, Canada, assignor to The Lubrizol orporation, a corporation of ilhio No Drawing. Filed Apr. 8, 31955, Ser. No. 500,237

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quantities to an aqueous acidic zinc phosphate solution containing an accelerating agent basically alters the crystal texture of the particles which make up the resultant coating from the conventional crystalline form to much smaller ball or amorphous-like grains. It has been further found that this coating is obtained, however, only when certain proportions of calcium and zinc are present in certain critical ratios, and these critical ratios are maintained concurrently with certain other critical ratios of the oxidizing agent relatve to the phosphate ion in the operating or coating solutions. 7 v g In the metal coating art it has long been understood that aqueous acidic zinc phosphate solutions continue to form coatings on metallic surfaces only so long as the coating solution is maintained within certain ranges of acidity, phosphate content, free acid content, etc. or, in other words, is in balance. The introduction of the calcium ion into the solution has been found to alter the equilibrium conditions of thesolution which constitute balance. While the, actual proportions of the various ingredients may vary considerably, the relative proportions are more critical and should be maintained in order to assure the continuous formation of amorphous-like zinc phosphate coatings.

The aqueous, acidic zinc phosphate operating solution may satisfactorily contain zinc in an amount in the range of about 0.5% to 1.5% and calcium in an amount in the range of about 0.012% to'about 7.5%. For any particular actual concentration of calcium and zinc the ratio of calcium to zinc should be between 1 calcium to 4 zinc, and 5 calcium to l zinc, or otherwise stated, the ratio of calcium to zinc should extend from 0.25 to 5.0 and preferably be in the range of 0.6 to 4.0. The solutions should contain as an oxidizing agent either a nitrite or a chlorate. When nitrite is used as this oxidizing agent the ratio of nitrite to phosphate may extend from about 0.01 to about 0.05; when chlorate is used the ratio of chlorate to phosphate may extend from about 0.05 to about 2.0.

When the above conditions are observed and the solutions are operated at a point strength between about and 60, points being the number of ml. of N/l0 NaOH which are required to neutralize a 10 ml. sample of the solution to a phenolphthalein end point, the resultant coatings are amorphous in appearance rather than crystalline. Moreover, the solution will coat and produce such coatings on iron, steel and zinc continuously and for so long 3,lhl,549 Patented es. 115,- i964 as the depleting ingredients are properly replenished to maintain the above given relative proportionsof ingredi ents. Similar coatings are formed on aluminum, but they are extremely thin and difficult to detect by visual inspection. The unusually line and uniform texture of the resultant coatings makes them especially advantageous for use asa base for, paint, and particularly as a base for heretofore hard-to-bond silicone paints. The coatings are additionally useful as an aid in increasing electrical resistance of a metallic member, in applications requiring high heat resistance and as an aid in metal deformation operations. In metal deformation applications the coatings may be used with or without the usual lubricating agents, as desired. a

The operating solutions of this invention may be made in the conventional manner using conventional zinc acid phosphate solution makeup chemicals. The desired calcium content may be obtained by adding calcium to the solution or to a concentrated makeup material in a variety of this available forms including calicum oxide, calcium hydroxide, calcium nitrate, calcium chloride, calcium hydrogen phosphate, calcium dihydrogen phosphate, etc. Broadly, the calcium may be introduced in the'form of any salt which is soluble in the solution and the anion portion of which is not detrimental to the coating-forming ability of the solution.

The oxidizing agents may be introduced in any of their conventional forms which are now well known to those skilled in the art.

The superior, uniform, fine grain-size coating of this invention may be obtained by operating the solutions of this invention over a relatively Wide range of temperature conditions. The solutions may be applied by spraying to the work being treated, or the work may be immersed in the solution as desired. Satisfactory results have been obtained from the operation of the solutions of this invention at temperatures in the range of about F. to slightly above 200 F.

From the continuous operation of the solutions of this invention, it has been determined that only minor proportions of calcium are deposited from the solution and become a part of the protective coating. The proportions of each of the replenishing materials which are satisfactory to maintain the above stated relative proportions of calcium, zinc, and oxidizing agent in the processing solution has been found to be widely variant from the amounts and proportions of these materials which are,

employed in the original make-up of the solutions. For the operating solutions above defined, it has been found that continuous operation is assured if the replenishing material contains calcium and zinc in the ratio of calcium to zinc in the range of 0.06 to 1.4 and preferably in the range of 0.1 to 0.65; the replenishing material should also contain zinc and P0 ion in the ratio of zinc to P0 in the range of 0.2 to 0.4 and preferably in the range of 0.22 to 0.30. When these proportions of calicum, zinc and P0 are provided in the replenishing material, the replenished solutions operate satisfactorily over a wide range of ratio of oxidizing agent to. P0 ion. In general, it is desirable to provide a specific replenishing composition which will produce the highest calcium concentration in the operating solution which can be maintained in equilibrium as work is processed through the solution. This may be accomplished by selecting a ratio of calcium to zinc in the replenishing material toward the high end of the range. The replenishing material preferably will contain chlorate, where it is the oxidizing agent used as one of its ingredients. It has been found that the best coating forming conditions and the best balance of the processing solution are obtained when the proportion of oxidizing agent is regulated relative to the calcium content of the replenishing material. In general, in the replenishing material, the proportion of calcium should increase as the proportion of chlorate is increased. Other anions than C10 may be present which do not detrimentally affect the coating or coating-forming ability of the solution, and calcium should increase as the proportion of such other anions in combination with the chlorate increases. The preferred proportion of chlorate in the replenishing material is that proportion which will provide a ratio of Ca/ClO in the range of about 0.1 to about 0.3.

Where nitrite is used as the oxidizing agent it is not feasible to incorporate it in the replenishing material directly for reasons of incompatibility, but the same considerations apply relative to quantity even though the nitrite is added to the operating solution separately. For example, when nitrite is used in the proportions above set forth, it will be understood that additional substantially innocuous anions such as chloride ions may be present to permit the indicated concentration of calcium otherwise be present, and the ratio of calcium to the sum of the chloride and chlorate or nitrite anions should be in the range of about 0.15 to about 0.5. When these general guides are followed, it has been found that the operating solution is provided with the most effective concentration of calcium which can be maintained in balance. A number of specific replenishing compositions which have been found to be satisfactory for use in connection with a variety of specific operating solutions are set forth in the examples which are given below.

The examples which follow are intended to illustrate in greater detail typical concentrations of ingredients in operating solutions, typical operating conditions and satisfactory replenishing materials for the solutions, but are not intended to limit the invention to the specific ingredients or proportions therein illustrated.

Example 1 An operating bath was prepared and upon analysis found to contain the following ingredients:

Percent (weight/volume) Zinc 0.42 Calcium 0.16 P 1.06 N0 0.68 C 0.32 Ca/Zn0.38.

NO /PO 0.64. ClO /PO -0.30. F.A.-3.5. T.A.-21.2. Balance water.

A satisfactory material for replenishing this operating solution is as follows:

Percent (weight/volume) Zinc 9.8 Calcium 1.8 Po. 38.4 N0 9.1 C10 7.2

Specific gravity-1.50.

Zn/PO.,0.255.

Balance water.

H PO acidity, percent by weight, 60 grams/liter9.8%. H PO acidity, dry basis22.2%.

Balance water A satisfactory material for replenishing the above operating solution is as follows:

Percent (weight/ volume) Zinc .8 Calcium 1.8 P0 38.4 N0 9.1 C10 7.2 Specific gravity-1.50.

Zn/PO -0.255. Ca/NO +ClO 0.11. Balance water.

Similar results are obtained from the use of the above processing solution altered so that the N0 and C10 were replaced, in separate solutions, by: 0.03% N0 0.2% sodium metal nitro benzene sulfonate; 0.008% H 0 and 0.1% dinitrobcnzene sulfonate.

An operating solution was prepared having the following analysis:

Example 3 Percent (weight/volume) Zinc 0.30 Calcium 0.31 PO, 0.68 Nitrate 1.04 Chlorate 0.27 Ca/Zn1. NO /PO -1.53. F.A.-2.6. T.A.17.1.

Balance water.

A satisfactory replenishing material for this bath is the replenishing solution given in Example 4.

Mild steel tubing was preliminarily cleaned and immersed in the above solution at F. for three minutes and withdrawn. The tubing emerging from the solution was found to have on its surface a uniform, dense, hard coating. The tubing was drawn through drawing dies at a reduction of 40% and the surface upon inspection was found to be unusually uniform and clean. The above bath was used for the processing of over 50,00 sq. ft. of tubing and the quality of the coating and the condition of the surface after drawing remained uniformly high for the entire batch.

What is claimed is:

1. A solution for forming a protective phosphate coating on the surfaces of iron, steel, zinc and aluminum, which comprises an aqueous acidic solution consisting essentially of about 0.05% to about 1.5% zinc ion, about 0.012% to about 7.5% calcium ion, the weight ratio of calcium ion to zinc ion being in the range of 0.25 to 5.0, the phosphate ion, and the nitrite ion, the weight ratio of nitrite to phosphate being in the range of 0.01 to about 0.05.

2. A solution for forming a protective phosphate coating on the surfaces of iron, steel, zinc and aluminum, which comprises an aqueous acidic solution consisting essentially of about 0.05% to about 1.5% zinc ion, about 5 0.012% to about 7.5% calcium ion, the weight ratio of calcium ion to zinc ion being in the range of 0.25 to 5.0, the phosphate ion, and the chlorate ion, the Weight ratio of chlorate to phosphate being in the range of about 0.05 to about 2.0.

References Cited in the file of this patent UNITED STATES PATENTS 6 Cliiford ct a1 May 8, 1945 Snyder Aug. 16, 1949 Rosenbloom Feb. 28, 1950 Amundsen Feb. 6, 1951 Henricks Mar. 4, 1952 Thome-Johannesen May 20, 1952 Russell Nov. 22, 1955 FOREIGN PATENTS Germany July 1, 1922 Germany Nov. 20, 1943 Great Britain Dec. 23, 1935 

1. A SOLUTION FOR FORMING A PROTECTIVE PHOSPHATE COATING ON THE SURFACES OF IRON, STEEL, ZONE AND ALUMINUM, WHICH COMPRISES AN AQUEOUS ACIDIC SOLUTION CONSISTING ESSENTIALLY OF ABOUT 0.05% TO ABOUT 1.5% ZINC ION, ABOUT 0.012% TO ABOUT 7.5% CALCIUM ION, THE WEIGHT RATIO OF CALCIM ION TO ZINC ION BEING IN THE RANGE OF 0.25 TO 5.0, THE PHOSPHATE ION, AND THE NITRATE ION, THE WEIGHT RATIO OF NITRITE TO PHOSPHATE BEING IN THE RANGE OF 0.01 TO ABOUT 0.05. 